Tankless Water Heater Not Working? Common Problems and What Causes Them

Tankless units are reliable, but they behave differently from tanks, and some of their quirks look like faults when they are not. Here is how to tell the difference.

Before calling anyone, it helps to know whether you are seeing a real fault or normal tankless behavior. If it is a fault, RapidHeat Stockton can connect Stockton-area homeowners with a local professional for tankless water heater repair. Either way, start with your owner's manual, because error codes and reset steps differ by brand.

No Hot Water at a Low Flow

A tankless heater only fires when water flows faster than a minimum rate. A hot tap barely cracked open, or a low-flow fixture, may not reach it, so the unit never turns on. Open the tap further. If hot water then arrives, the unit is working as designed.

The "Cold Water Sandwich"

You get hot water, then a short burst of cold, then hot again. It happens when someone turns hot water off and back on: hot water left in the pipe comes out first, then cold water passes through while the unit relights. It is a normal trait of many tankless systems, not a failure. Some models have features or add-ons to reduce it.

Hot Water Turns Lukewarm When Two Fixtures Run

Every tankless unit has a maximum flow at a given temperature rise. Running two showers and a washing machine together can exceed it. In winter, when incoming water is colder, the limit is reached sooner. This is a sizing issue rather than a fault.

Ignition Failure

On gas units, the heater tries to light and gives up, usually with an error code. Common causes:

• The gas supply valve is partly closed
• The gas line is too small for the unit's input, a frequent problem when a tankless unit replaced a tank on the same line
• A dirty or failed flame rod or igniter
• Venting or combustion air problems

Gas line sizing and venting are not homeowner jobs. Note the error code and call a professional.

Flame Failure or Shutdown During Use

If the unit lights and then shuts off, look for a blocked vent termination outside, such as a bird nest, leaves or something stored against it. Also check that the gas supply was not interrupted. A unit that shuts down repeatedly needs a diagnosis.

Scale Buildup

Minerals in hard water coat the heat exchanger. Signs include temperature swings, lower hot water flow and scale-related error messages on some models. The fix is descaling on the schedule in the manual, often about once a year in harder water. Your water provider's yearly quality report lists hardness.

Clogged Inlet Filter

Most units have a small screen on the cold water inlet. Debris on it reduces flow and can stop the unit firing. Cleaning it is usually part of routine maintenance; check the manual for how to reach it safely.

Error Codes

Each brand uses its own codes. Write down the exact code, look it up in the manual or the manufacturer's website, and follow the listed steps. If a code returns after a reset, stop resetting it and get the unit checked.

Freezing

Hard freezes are uncommon around Stockton, but outdoor-mounted units and exposed pipes can still freeze on a cold night. Most outdoor units have built-in freeze protection that only works when they have power and, on gas models, gas. Do not unplug an outdoor unit in winter.

What to Note Before Calling

• Brand and model from the rating label
• Any error code shown
• When the problem happens: one tap, several taps, all the time
• When it was last descaled

The Department of Energy expects tankless units to last more than 20 years with proper care, according to its tankless water heater guide, so a repair on a working-age unit is usually worth considering.